Placing images in RMarkdown documents can be difficult. Here are two approaches.
Easiest if image is in same directory as your RMarkdown file.
Here I use the code:

{ width=20% }
Note the space before and after the
{and}.
This method uses code in an R chunk. You could set echo = FALSE to hide this chunk in the final RMarkdown document.
I use the outwidth = option to set the size of the image: {r out.width = "20%"}
knitr::include_graphics("Sesame-Street-optimize.gif")